1. Field of the Invention
The present invention relates to a mandrel mill used to produce seamless tubes, more particularly seamless steel tubes, and a method of tube rolling by using the mandrel mill.
2. Description of Related Art
A method of using a mandrel mill is available as a method of producing seamless steel tubes. In this method, as shown in FIG. 1, after a billet 11 is heated by a heating furnace 12, it is pierced by a roughing-down mill 13 called a piercer to form a hollow shell. Next, the hollow shell is elongated and rolled by a following mandrel mill 14, and finished to a predetermined wall thickness. After reheating, the hollow shell is processed by a reducer mill 15 to a predetermined outer diameter, thereby obtaining a seamless steel tube as a product. The reheating process after elongation and rolling my be omitted sometimes.
The mandrel mill 14 has four to eight two-roll stands arranged in a row along a pass line, each stand being provided with a pair of caliber rolls. Between two stands adjacent to each other, the roll gap adjustment direction of the caliber rolls on one of the stands is set crosswise and shifted 90 degrees from the roll gap adjustment direction of the caliber rolls on the other stand in a plane perpendicular to the pass line. The hollow shell then passes between the caliber rolls of each stand, with a mandrel bar inserted therein, and is rolled in this passing process.
In the tube rolling by using such a mandrel mill, the wall thickness of the tube is finished to a predetermined dimension by rolling the material in a gap between the caliber rolls and the mandrel bar. For this reason, if the wall thickness at the finishing stand is different, the gap dimension between the caliber rolls and the mandrel bar needs to be changed accordingly. As methods of changing the gap dimension, three methods are available: replacing the mandrel bar, replacing the caliber rolls and changing the roll gap by adjusting the roll positions.
However, replacing the caliber rolls requires more effort than replacing the mandrel bar. In case the roll gap is changed by adjusting the roll positions, an eccentric wall thickness may occur in the circumferential direction of the material to be rolled. This is explained as follows. Since the wall thickness is determined by the gap determined by the caliber diameter of the caliber roll and the outer diameter of the mandrel bar, the caliber shape formed by a pair of caliber rolls is changed when the roll gap has a value other than a predetermined value, thereby changing the gap in the circumferential direction.
FIGS. 2A and 2B are schematic views showing this phenomenon by taking a truly round caliber as an example. FIG. 2A shows a state wherein the gap between a pair of caliber rolls 3', 3' and a mandrel bar 5 is uniform in the circumferential direction, namely, the wall thickness is uniform in the circumferential direction. From this state, the gap between the caliber rolls 3', 3' and the mandrel bar 5 is changed as shown in FIG. 2B. At the same time, the gap becomes nonuniform in the circumferential direction, thereby causing an eccentric wall thickness in a rolled material in the circumferential direction.
Because of these reasons, it is customary to replace the mandrel bar to change the wall thickness at the finishing stand. However, since the rolling schedule has been determined so that the wall thickness of a material to be rolled is changed in 0.5 mm increments, it is necessary to prepare mandrel bars having outer diameters in 1.0 mm increments. In addition, when rolling hollow shells having a wall thickness, about 15 mandrel bars are usually necessary, since a mandrel bar is cooled after extracted from a hollow shell and is subjected to a process wherein lubricant is applied thereto for the next rolling. For this reason, it is necessary to retain a great many mandrel bars.
To solve this problem, a four-roll stand was conceived. This stand having a combination of four caliber rolls is disposed as the final stand of the row of the stands. The roll gap adjustment direction at the stand was shifted 45 degrees from the roll gap adjustment direction at the preceding two-roll stand. The mandrel mill having the above-mentioned structure is detailed in Japanese Patent Application Mid-open No. Hei 6-87008.
With this mandrel mill, any eccentric wall thickness generated when the roll gap is changed at the row of the two-roll stands used to reduce the wall thickness is canceled by the four-roll stand as the final stand. With this feature, gap change is possible in a wider range at the row of the two-roll stands. As a result, the number of the types of mandrel bars can be decreased. The mandrel mill similar to the above-mentioned one is also described in the Japanese Patent Application Mid-open No. Sho 62-28011.
However, in case the number of caliber rolls is increased at a caliber roll stand, the width of each roll is inevitably made smaller, and a phenomenon called "squeezed outward" wherein the material is squeezed outward from between the rolls is apt to occur,
Consequently, in the case of a mandrel mill provided with a four-roll stand as the final stand of the row of stands, the problem of "squeezed outward" occurs at the four-roll stand. This problem of "squeezed outward" cannot be prevented by outer diameter adjustment by using a reducer mill and muses the quality of seamless steel tubes, that is, products of the mandrel mill, to deteriorate.
Accordingly, bemuse of the secondary defect, namely, deterioration in quality, it cannot be said that the conventional mandrel mill, which is provided with a four-roll stand as the final stand of the row of stands to decrease the number of the types of mandrel bars, is satisfactory.